Multivalent ions induce lateral structural inhomogeneities in polyelectrolyte brushes
Dr. Yu Jing is an assistant professor in the School of Materials Science and Engineering (MSE) at Nanyang Technological University (NTU) in Singapore. He obtained his Bachelor’s degree from Tsinghua University in 2007, and PhD from the University of California, Santa Barbara in 2012, both in Chemical Engineering. His PhD research under the supervision of Prof. Jacob Israelachvili focused on the nanomechanics of biomaterials and biomimetics. He conducted experimental and theoretical studies on both mussel and gecko adhesive systems, and developed bio-inspired polymer surfaces and nano-structured materials. In 2014, he expanded his research interests towards polyelectrolytes brushes during his postdoc with Prof. Matthew Tirrell at the University of Chicago.
Understanding the response of polyelectrolytes to their environment has challenged materials scientists for decades. Polyelectrolyte brushes are of particular interest, as they can regulate transport of ions and molecules, control wettability and adhesion of surfaces, or convert chemical and biochemical stimuli into optical, electrical, and mechanical signals. We combinedneutron-reflectivity (NR), atomic force microscopy (AFM), surface forces apparatus (SFA) measurements, and coarse-grained molecular dynamics (MD) simulations to study the structure of planar biomimetic polyelectrolyte brushes (poly(styrenesulfonate), PSS) in a variety of solvent conditions, especially in the presence of multivalent counterions. Multivalent-ion induced lateral structural inhomogeneitiesof the PSS brushes were first indicated by NR measurements. AFM images provide a direct visualization of lateral inhomogeneities on the surface of polyelectrolyte brushes collapsed in solutions containing trivalent counterions. These images are interpreted in the context of a coarse-grained molecular model, and are corroborated by accompanying interaction-force measurements with the SFA. Our findings indicate that lateral inhomogeneities are absent from PSS brush layers collapsed in a poor solvent without multivalent ions. The inhomogeneous structures of the polyelectrolyte brushes in the presence of multivalent ions can dramatically alter their functionalities in various biomedical applications, such as lubrication, sensors, and DNA-based microarrays.